Let's turn that on its head... I have a theory that the Switch was built to serve Japanese interests first. Handheld gaming is much stronger over there, so the flexibility of the Switch is ideal to Japan. Having the "guarantee" of strong Japanese sales, with no real competitor, means a strong sales base even before western sales come into play. This is better displayed in the 3DS' >60 million units sold.
So, if the Switch builds up appreciable sales in Japan (which is more likely than not) that alone can translate into a far larger worldwide userbase than the Wii U, and a larger userbase means more support which in turn reinforces value in the west.
Basically, I think the Switch's hybrid concept is built for Japan first because that sales momentum will prevent it from being another Wii U, even if it doesn't hit 3DS levels (which it absolutely still could, no way to know yet).
I agree with that theory to a point. Japan is kind of a trap. It's tastes are so different than the rest of the world that the more you target that market, the more you alienate the larger worldwide market. The Switch makes perfect sense for Japan where the handheld mostly is the main console. With no competition from the console market in Japan, the Switch is all upside. They get a more console like handheld experience and can switch to true console type play if they want to.
The problem comes from markets with a strong console user base. There the Switch is inferior to those consoles in the home and is not likely to have much 3rd party support, yet doesn't cost appreciably less . If someone in that market is looking to get their first 8th gen console this Christmas now that the prices have dropped below $300, the Switch doesn't look like such a good deal.
Then for the mobile market there could potentially be problems due to its larger size making it less portable, and battery life being less due to being more powerful. If someone is already carrying a larger sized phone which have caught on more now, how willing are they to want to carry around an ever larger dedicated gaming device? I can see it for specific occasions where you know you'll have a lot of down time, but I don't see it being a normal habit.
All of these issues come about because of the Switch's need to make compromises in order to work as a hybrid device. In markets like Japan that are primed to want a hybrid device, the Switch should do well because it is perfectly aligned with their needs. On the other hand, in markets that have distinct console and handheld markets, the Switch won't compare favorably to dedicated devices in those markets.
Finally, I don't see the Switch as a Wii U replacement. I see it as a 3DS replacement. It's a mobile device that can send its display to the TV screen. However, if you want to view it as taking over both markets, then it would have to do the sales of both markets combined. Is anyone predicting that the Switch will do the sales of 3DS + Wii U? If not, then viewing the Switch as the next 3DS only makes more sense.